1. Field of the Invention
The present invention relates to an ink jet head and a method of producing the ink jet head. More particularly, the present invention is concerned with an ink jet head of the type which employs electro-thermal transducers as elements for generating discharge energy. Still more particularly, the present invention pertains to an improved construction for keeping a heater board having the electro-thermal transducers formed thereon and a top plate of the ink jet head in close and tight contact with each other at their joint surfaces.
2. Description of the Related Art
Hitherto, an ink jet head has been known in which electro-thermal transducers are used as the elements which generate energy for discharging ink. In this type of ink jet head, a wiring layer is formed on an exothermic resistor layer so that pairs of electrodes electrically connected to the exothermic resistors are formed. In operation, ink in each of a plurality of channels is heated by the heat from the associated exothermic element formed between the exothermic resistor layer and the electrodes, so that a bubble of ink is generated in the channel so as to eject an ink droplet from the ejection opening leading from the channel, thereby performing printing.
One form of the ink heat head of the type described is fabricated by separately preparing a substrate (referred to also as "heater board") on which the plurality of exothermic resistors are formed and a top plate in which a plurality of grooves are formed, and the heater board and the top plate are joined with each other to make close contact with each other so that ink channels (referred to also as "nozzles") corresponding to the exothermic resistors are formed by the grooves in the top plate.
A method of fabricating an ink jet head of the type described, disclosed in U.S. Pat. No. 5,095,321, employs a heater board as a first member having electro-thermal transducers formed thereon and a top plate as a second member. The second member is placed on the first member such that the ink channels are positioned right above the wirings of the electro-thermal transducers, and pressure is applied by a pressing device such as a spring so as to keep the first and second members in close contact with each other.
More specifically, as shown in FIGS. 7 and 8, a top plate 1300 is juxtaposed to a heater board 100 such that the ink channels are aligned with the electro-thermal transducers, and a pressing device such as a device incorporating springs applies pressure load to the top plate 1300 so as to keep these two members in close and proper contact with each other.
This construction is adopted in order to eliminate problems which may be caused when an adhesive is used. Namely, when an adhesive is used so as to adhere the top plate to the heater board, the adhesive may be spread and squeezed from the gap between these two members so as to fill or contaminate the minute ink channels, with a result that the ink is blocked or safe injection of the ink is hampered. In most cases, the top plate 1300 is made of a synthetic resinous material, because such a material provides a high efficiency of production and because the use of such a material eliminates any risk of rupture of the heater board 100.
The described known structure, however, suffers from a disadvantage in that it is extremely difficult to keep the initial high degree of flatness of the ink channels constituting the nozzle array, because once formed, the top plate 1300 tends to warp due to characteristics peculiar to resinous materials.
More specifically, FIG. 6 illustrates a top plate 1300 warping such that it is convex upward at mid portion of the nozzle array above the level of both end regions of the nozzle array. When this top plate 1300 is juxtaposed to the heater board 100 and pressed onto the heater board with an adequate level of pressure, close contact between the top plate 1300 and the heater board 100 cannot be obtained at the middle portion of the nozzle array, allowing crosstalk between the adjacent nozzles, i.e., release of ejection energy from one nozzle to another, failing to provide high accuracy in the ejection of ink, resulting in serious degradation in the quality of the printing.
It might be possible to eliminate this problem by applying pressure of a level exceeding the above-mentioned adequate level so as to forcibly straighten and level the top plate. Application of such a high pressure, however, tends to cause deformation and rupture of the ink channels in the region where the top plate 1300 contacts the heater board, with the result that the cross-sectional shapes of such ink channels are deformed so as to significantly impair the quality of the print.
Thus, warping of the top plate formed from a synthetic resin is one of the factors which reduce the yield of the ink jet head.